The present invention relates to communication systems, and more particularly, to decoders for decoding signals that have distortions introduced by electrical transmission lines.
Over the years, the rate of data transmission has displayed an ever-increasing trend. At present, optical systems are routinely capable of transmitting at rates of 40 Gbps and above, while electrical systems are approaching speeds up to 10 Gbps. The advantage provided by the higher transmission rates of optical systems, however, is sometimes offset by the lower costs involved in setting up an electrical transmission system. In addition, electrical transmission systems are more energy efficient than optical systems. The higher power requirements of optical systems often requires that these systems have larger physical dimensions to provide cooling. Hence, in many applications in which the very high data rates of optical systems are not needed, electrical systems are still favored.
Accordingly, increasing the transmission rate of electrical systems still further would be advantageous. One of the factors affecting the speed of electrical systems is the phenomenon known as the skin effect. The penetration depth of an electromagnetic(EM) wave into a conductor is dependent on the frequency of the EM wave. The higher the frequency, the shallower the penetration into the conductor. As a result, when sending a signal down a conductor with a given physical cross-sectional area, the effective area seen by the higher frequency components is smaller than that seen by the lower frequency components. Accordingly, the higher frequencies experience higher impedance than the lower frequencies. This frequency dependent impedance is known as the skin effect.
Data streams can be modulated on an electrical conductor using a number of different chemes. The two most common schemes are return to zero (RZ) and non-return to zero NRZ). In RZ modulation, the modulation of each bit of information begins and ends at the same voltage level, which is usually ground. Hence, a transmission consisting of a string of N Is ideally appears as a string of N individual pulses having a width of one half time period each and a voltage V. In NRZ schemes, the modulation level is not returned to the same level at the end of each bit. In an NRZ transmission scheme, the string of N Is would ideally appear as a single pulse that is N time periods long with a voltage V.
In NRZ transmission schemes that suffer from skin effect, the rise and fall times of the pulses is increased due to the loss of energy at the high frequencies. If the rise and fall times are long compared to the time period allocated for each bit, the potential to which the signal rises on the conductor will depend on the bit pattern. This bit pattern dependence can lead to bit errors when the signal is decoded at the receiving end of the transmission line.
Prior art systems attempt to overcome this problem by adjusting the data stream signal at either the transmission or reception end of the communication channel. At the transmission end of the channel, these systems alter the pulse shapes to provide more energy in the high frequency components thereby compensating for the loss of high frequency information during transmission.
This type of system requires that additional high frequency energy be provided to compensate for the eventual loss through the communication medium. This additional high-frequency signal energy exacerbates cross talk at the sending end of the communication link. In addition, increasing the high frequency energy content at the transmitter results in higher power consumption.
The second class of equalization solutions operates by providing an inverse skin effect filter at the reception end of the transmission line. Such a filter must have a large gain at the high frequencies to make up for the lost energy in the communication channel. At very high frequencies, high gain filters of this type with sufficiently low noise are difficult to construct.
Broadly, it is the object of the present invention to provide an improved data decoding system.
This and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is a decoder for decoding a signal. The decoder includes a discriminator and a threshold generator. The discriminator receives the signal and generates an output voltage equal to a first voltage if the signal is less than a threshold level that is input to the discriminator and equal to a second voltage if the signal is greater than the threshold level. The threshold level depends on the output from the discriminator in a preceding time interval that depends on the impulse response of a transmission link through which the input signal has passed. The threshold generator implements a low-pass analog filter that receives the output voltage during each of the clock periods and generates therefrom a filtered output signal. The threshold generator may also implement a variable gain amplifier for amplifying the filter output signal by an amount specified by a gain input signal to generate the threshold level. The gain input signal depends can be set with reference to a timing signal indicative of whether the input signal crossed the threshold level at a time that was early or late relative to the beginning of a clock period. The gain input signal during one of the clock periods preferably depends on the timing signal and the output of the discriminator during two clock periods prior to that clock period. The threshold generator may also include a circuit for offsetting the threshold level by an amount determined by the timing signal and the previously generated output voltage values.